1. Field of the Invention
The present invention relates generally to an improvement in soldering, and more particularly to a method of and apparatus for applying molten solder onto a desired point of a workpiece under well-controlled condition.
2. Prior Art Statement
Japanese patent publication No. 34209/1971 discloses a soldering apparatus for applying molten solder onto a desired point or area of a workpiece, such as a printed circuit board, wherein a conical pot is used for melting a solid solder. A rod-shaped solder is cut and fed to the conical pot to be melted therein and the molten solder is allowed to drop through a hole provided at the bottom of the pot. However, this type soldering apparatus has a problem that some portion of the solid solder drops through the hole in the unmelted state while the other portion of the solder is dropped in the overheated condition, since it is impossible to retain the whole solid solder for a constant time, resulting in uneven and unreliable soldering.
Unexamined Japanese Utility Model Publication No. 77427/1975 discloses a soldering apparatus having a conical melting pot provided with a needle valve at the bottom of the pot. In this known apparatus, a molten mass of solder is contained in the conical melting pot, and a constant volume of the molten solder is allowed to drop through the needle valve. However, due to surface tention of the molten solder, it becomes impossible to control the volume of dropping molten solder precisely particularly when the volume of molten solder to be applied onto the workpiece is relatively small as is the case where the workpiece is a printed circuit board.
In order to solve the aforementioned problems of the known apparatuses, the inventor of this invention has previously proposed an improvement in soldering in my co-pending Japanese Patent Application No. 110505/1986. According to my previous proposal, a solder melting pot is formed by plural trowel members which engage with each other to hold a solid solder tip for a pre-set time to melt the same and then they are separated to allow the molten solder to drop onto a desired point or area of a workpiece. However, the velosity of the solid solder tip supplied to the solder melting pot cannot be controlled so that it becomes hard to control the temperature of the molten solder in the solder melting pot precisely.
Another probelm of the system according to my previous proposal is the residing flux or other impurities sticking on the end portions, particularly on the engaging end faces, of the trowel members, which lower the heat transfer from the trowel members to the solder significantly to cause change in temperature of the molten solder. Such a change in molten solder contained in the solder melting pot leads to incomplete or unstable soldering.